Very hard... and helpful!

The GMAT club tests are worth the hype. There are a lot of really great things about them: (i) the number! There are 26 tests (1,500+ questions), which is a TON. Each question has a detailed explanation, stats from others who attempted the same question (timing, accuracy, etc.), and a link to the forum thread discussing the question. (ii)- difficulty. These tests are tough! Much more so than the real thing. Safe to say that if you can do well on these tests, you’ll have no problem at all with the actual. (iii) the support. Bunuel, bb, and team have done an excellent job creating and curating these questions and the tests. If you are ever having any technical issues with the tests, they are there to help (and fast too!)

In summary- highly recommended. BTW: you can get access to these tests by redeeming 25 kudos or by using GMAT Club points. Also, they open up access to the public during holidays also! (obviously not ideal to be doing math tests during a holiday weekend, but it’s still nice!)